Silicon powder for preparing alkyl- or aryl- halogenosilanes

ABSTRACT

The invention concerns a silicon powder for making alkyl- or aryl-halogenosilanes, with particle-size distribution less than 350 μm, and containing less than 3% and preferably less than 2% of particles having a size less than 5 μm. Said powder enables to improve efficiency of synthesis reaction.

FIELD OF THE INVENTION

[0001] The invention relates to a silicon or silicon alloy powder, of aparticle size less than 350 μm, specially adapted for the manufacture ofalkyl- or aryl-halogenosilanes, intended for silicone synthesis.

STATE OF THE RELATED ART

[0002] The synthesis of alkyl- or aryl-halogenosilanes by means of areaction between 250 and 350° C. of a halogenated hydrocarbon, forexample methyl chloride, on silicon has been known since the patent U.S.Pat. No. 2,380,995 granted in 1945 to E. G. ROCHOW.

[0003] This reaction has achieved significant industrial development forsilicone manufacture; it is frequently conducted in a fluidised bedreaction vessel with silicon in powder form, most frequently in particlesizes less than 350 μm. For many years, it has been usual to use afraction with a particle size between approximately 50 and 350 μm, thepresence of silicon particles less than 50 μm in size being a cause ofmaterial loss and decrease in reaction vessel yield. An illustration ofthe use of such a particle size distribution can be seen for example inthe patent EP 0191502 held by Union Carbide, filed in 1986, whichrecommends a distribution between 48 mesh (300 μm) and 325 mesh (45 μm),or the application EP 0893408 by Pechiney Electrométallurgie, filed in1998, which specifies, in examples 1 and 2, a 50-350 μm distribution.

PURPOSE OF THE INVENTION

[0004] The invention relates to a silicon or silicon alloy powder forpreparing alkyl- or aryl-halogenosilanes, of a particle size of lessthan 350 μm, comprising a fraction of particles having a size less than5 μm of less than 3%, and preferentially less than 2%, by weight.

DESCRIPTION OF THE INVENTION

[0005] The invention is based on the observation by the applicant of thepresence in silicon powders screened to obtain a particle sizedistribution of around 50 to 350 μm, of non-negligible quantities ofparticles having a size less than 5 μm. Unexpectedly, experience showsthat screening a powder to extract the fraction less than 50 μm provesto be ineffective in eliminating the finest particles, for example thefraction less than 5 μm. These very fine particles are probablygenerated during the packing of the product and the observation of thepowder under a microscope confirms their existence.

[0006] The evaluation of their relative quantity by weight can bedetermined by means of laser granulometry; in silicon powders,irrespective of their preparation method, fractions of particles havinga size of less than 5 μm of around at least 4% by weight are alwaysfound. The applicant also observed that eliminating or reducing thecontent of these very fine particles made it possible to improve theRochow reaction yield. Therefore, the invention consists, in order touse the silicon powder-based contact mass, which represents asignificant proportion of the production cost of halogenosilanes, asefficiently as possibly, of reducing the content of particles having asize less than 5 μm to less than 3%, and preferably less than 2%.

[0007] To obtain this result, it is possible to use washing with waterof the powder ground to less than 350 μm, and screened if required toobtain a particle size distribution of 50-350 μm. This washing isfollowed by selective decantation, and then drying of the decantedpowder and also a vacuum draw-off to facilitate the removal of thewater. This technique makes it possible to obtain a strict particle sizedistribution at 5 μm, the final proportion of the residual fractionhaving a size less than 5 μm possibly reaching 0.5%.

[0008] It is also possible to use, for the selective elimination of thefinest particles, dispersion of the powder in a gas stream at a moderatevelocity. The velocity of the gas is chosen as a function of the desiredcut-off threshold, always operating in the laminar flow state. For thegas, it is preferable to choose oxygen-depleted air for safety reasons.

EXAMPLES Example 1

[0009] A chemical grade metallurgical silicon meeting the requiredspecifications for halogenosilane application was prepared in anelectric arc furnace. The alloy was cast, solidified, and then ground toa particle size of less than 350 μm. Five samples of one kg of productwere taken.

[0010] This type of powder is generally tested on a unit devised toevaluate its performances. In order to do this, 40 g of the powder ismixed with a catalyst and the mixture is placed in a glass reactionvessel 30 mm in diameter equipped with a stirrer. A stream of gaseousCH₃Cl is sent via a sintered glass disk supporting the powder. The gasflow rate is kept constant at 3.6 10⁻³ m³/hr. After heating the reactionmedium and starting the reaction, the system is maintained at 300° C.After 12 hours of reaction, the mean flow rate obtained indimethyldichlorosilane is noted, along with the content of this productin all the reaction products.

[0011] For the evaluation of the particle size grade of the powder ofsample No. 1, two types of measurements were made:

[0012] laser granulometry;

[0013] a simplified test with reference to the test described above byworking on the test powder directly without adding catalyst, at ambienttemperature, with no heating, and by replacing the CH₃Cl gas bynitrogen.

[0014] The laser granulometry detected 5.5% (by weight) of fines havinga size less than 5 μm.

[0015] In the simplified test, after 12 hours of treatment, the productremaining in the reaction vessel was retrieved and weighed. Of the 40 gof the initial product, only 37.2 g remained, i.e. a loss of 7%.

Example 2

[0016] Sample No. 2 prepared at the start of example 1 was screened at50 μm to extract the fraction with a particle size distribution of 0-50μm. On the sample screened in this way, a laser granulometry measurementwas made and detected 4.5% of fines having a size less than 5 μm. 40 gof powder was removed to carry out the simplified test described inexample 1. After 12 hours of treatment, the product remaining in thereaction vessel was retrieved and weighed. Of the 40 g of the initialproduct, only 37.8 g remained, i.e. a loss of 5.5%.

Example 3

[0017] Sample No. 3, with a particle size distribution of less than 350μm, prepared at the start of example 1, was washed in 10 litres ofwater. The mixture obtained was then allowed to decant for one hour andthe supernatant liquor was then eliminated and the decanted powderretrieved and dried under an infrared lamp in a vacuum. On the powderwashed in this way, a laser granulometry measurement was made anddetected 0.5% fines having a size less than 5 μm.

[0018] On said washed sample No. 3, 40 g of powder was removed to carryout the simplified test described in example 1. After 12 hours oftreatment, the product remaining in the reaction vessel was retrievedand weighed. Of the 40 g of the initial product, only 39.7 g remained,i.e. a loss of 0.75%.

Example 4

[0019] Sample No. 4 prepared at the start of example 1 was dispersed inregular throws at a rate of 10 g per minute at the top of a tube 50 mmin diameter and 500 mm high, with an upward gas stream composed of onevolume of air and two volumes of nitrogen running through it, whereinthe flow rate was set to 60 cm³ per second.

[0020] The removal of a fine dust entrained with the gas was observed atthe top of the tube. On the powder retrieved at the base of the tube, alaser granulometry measurement was made and detected 2% fines having asize less than 5 μm.

[0021] On said sample No. 4, 40 g of powder was removed to carry out thesimplified test described in example 1. After 12 hours of treatment, theproduct remaining in the reaction vessel was retrieved and weighed. Ofthe 40 g of the initial product, 39.0 g remained, i.e. a loss of 2.5%.

Example 5

[0022] Sample No. 5 was screened at 50 μm to prepare a powder with aparticle size distribution of 50-350 μm, which was then used to repeatthe operation described in example 4.

[0023] On the powder retrieved at the base of the tube, a lasergranulometry measurement was made and detected 1% fines having a sizeless than 5 μm. On said sample No. 5 treated in this way, 40 g of powderwas removed to carry out the simplified test described in example 1.After 12 hours of treatment, the product remaining in the reactionvessel was retrieved and weighed. Of the 40 g of the initial product,39.4 g remained, i.e. a loss of 1.5%.

1. Silicon or silicon alloy powder for preparing alkyl- oraryl-halogenosilanes of a particle size of less than 350 μm,characterised by a fraction of particles having a size less than 5 μm ofless than 3% by weight.
 2. Silicon powder according to claim 1,characterised by a fraction of particles having a size less than 5 μm ofless than 2% by weight.
 3. Method to prepare a powder according to anyof claims 1 or 2, comprising grinding of the powder to a particle sizeless than 350 μm, washing with water, decantation and drying.
 4. Methodto prepare a powder according to claim 3, characterised in that thepowder ground to less than 350 μm is screened to 50 μm to obtain aparticle size distribution of 50-350 μm.
 5. Method to prepare a powderaccording to any of claims 1 or 2, comprising grinding of the powder toa particle size of less than 350 μm, screening to obtain a particle sizedistribution of 50-350 μm and dispersion of said powder in a gas streamin the laminar flow state.
 6. Method according to claim 5, characterisedin that the gas in oxygen-depleted air.